First of all, this doesn't seem to be any sort of security vulnerability (not related to any stack overflow or any stack smashing etc.).It is something to do with parsing.


Also, not related to glob_buffer or it being empty as suggested above. (even in normal case it is like that).

The problem is in com_server_help:

static int com_server_help(String *buffer __attribute__((unused)),
			   char *line __attribute__((unused)), char *help_arg)
{
  MYSQL_ROW cur;
  const char *server_cmd= buffer->ptr();
  char cmd_buf[100 + 1];
  MYSQL_RES *result;
  int error;
  
  if (help_arg[0] != '\'')
  {
	char *end_arg= strend(help_arg);
	if(--end_arg)
	{
		while (my_isspace(charset_info,*end_arg))
          end_arg--;
		*++end_arg= '\0';
	}
	(void) strxnmov(cmd_buf, sizeof(cmd_buf), "help '", help_arg, "'", NullS);
    server_cmd= cmd_buf;
  }
  
  if (!status.batch)
  {
    old_buffer= *buffer;
    old_buffer.copy();
  }
======

As you can see it explicitly checks for single quote and does some string filtering to finally append " help ' " and " ' " to it if does not have them already.   

The problem lies here -- 
  const char *server_cmd= buffer->ptr() 

If the string already starts with single quote, server_cmd ends up with value of glob_buffer like this:

 print server_cmd
$10 = 0x98d660 "Your MySQL connection id is 11\nServer version: 5.5.27-rel28.0-debug-log Built by raghavendra at Tue Aug 21 00:41:10 IST 2012\n"

and rest follows.

Interesting to observe that the argument has been marked __attribute__((unused)) but is still used. 

This section 
===
  if (!status.batch)
  {
    old_buffer= *buffer;
    old_buffer.copy();
  }
======

is also suspicious (because of unused attribute) but *not* directly relevant to this bug. (For curious, old_buffer is  used in com_edit when \e is invoked, however, after the fix(below) I checked and \e along with \h was working fine: something like 

 > select \h help 'contents' \e    will copy select to $EDITOR's buffer 

Anyways, here is the fix:

=== modified file 'Percona-Server/client/mysql.cc'
--- Percona-Server/client/mysql.cc      2012-08-07 06:10:00 +0000
+++ Percona-Server/client/mysql.cc      2012-09-05 16:14:14 +0000
@@ -2827,7 +2827,7 @@
                           char *line __attribute__((unused)), char *help_arg)
 {
   MYSQL_ROW cur;
-  const char *server_cmd= buffer->ptr();
+  const char *server_cmd= help_arg;
   char cmd_buf[100 + 1];
   MYSQL_RES *result;
   int error;
@@ -2842,8 +2842,10 @@
                *++end_arg= '\0';
        }
        (void) strxnmov(cmd_buf, sizeof(cmd_buf), "help '", help_arg, "'", NullS);
-    server_cmd= cmd_buf;
+  } else {
+       (void) strxnmov(cmd_buf, sizeof(cmd_buf), "help ", help_arg, NullS);
   }
+  server_cmd= cmd_buf;


After the fix:

>>./client/mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 19
Server version: 5.5.27-log Source distribution

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> help contents
You asked for help about help category: "Contents"
For more information, type 'help <item>', where <item> is one of the following
categories:
   Account Management
   Administration
   Compound Statements
   Data Definition
   Data Manipulation
   Data Types
   Functions
   Functions and Modifiers for Use with GROUP BY
   Geographic Features
   Help Metadata
   Language Structure
   Plugins
   Procedures
   Table Maintenance
   Transactions
   User-Defined Functions
   Utility

mysql> help 'contents'
You asked for help about help category: "Contents"
For more information, type 'help <item>', where <item> is one of the following
categories:
   Account Management
   Administration
   Compound Statements
   Data Definition
   Data Manipulation
   Data Types
   Functions
   Functions and Modifiers for Use with GROUP BY
   Geographic Features
   Help Metadata
   Language Structure
   Plugins
   Procedures
   Table Maintenance
   Transactions
   User-Defined Functions
   Utility


Also, with the test case:
=============================

./client/mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 20
Server version: 5.5.27-log Source distribution

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> create table t1 (`id` int(11) auto_increment, `name` varchar(255), primary key (`id`));
ERROR 1046 (3D000): No database selected
mysql> use test;
Database changed
mysql> drop table t1;
Query OK, 0 rows affected (0.01 sec)

mysql> create table t1 (`id` int(11) auto_increment, `name` varchar(255), primary key (`id`));
Query OK, 0 rows affected (0.00 sec)

mysql> INSERT INTO t1(`name`) VALUES ('test1'),('test2'),('test3'),('test4');
Query OK, 4 rows affected (0.00 sec)
Records: 4  Duplicates: 0  Warnings: 0

mysql> SELECT * FROM t1;
+----+-------+
| id | name  |
+----+-------+
|  1 | test1 |
|  2 | test2 |
|  3 | test3 |
|  4 | test4 |
+----+-------+
4 rows in set (0.00 sec)

mysql> DELETE FROM t1 LIMIT 1;
Query OK, 1 row affected (0.00 sec)

mysql> help 'contents'
You asked for help about help category: "Contents"
For more information, type 'help <item>', where <item> is one of the following
categories:
   Account Management
   Administration
   Compound Statements
   Data Definition
   Data Manipulation
   Data Types
   Functions
   Functions and Modifiers for Use with GROUP BY
   Geographic Features
   Help Metadata
   Language Structure
   Plugins
   Procedures
   Table Maintenance
   Transactions
   User-Defined Functions
   Utility

mysql> SELECT * FROM t1;
+----+-------+
| id | name  |
+----+-------+
|  2 | test2 |
|  3 | test3 |
|  4 | test4 |
+----+-------+
3 rows in set (0.00 sec)

mysql> \q
Bye
                                                                                                                                                                                                                                                   (origin/Percona-Server)~21:51-0
>>./client/mysql
Welcome to the MySQL monitor.  Commands end with ; or \g.
Your MySQL connection id is 21
Server version: 5.5.27-log Source distribution

Copyright (c) 2000, 2011, Oracle and/or its affiliates. All rights reserved.

Oracle is a registered trademark of Oracle Corporation and/or its
affiliates. Other names may be trademarks of their respective
owners.

Type 'help;' or '\h' for help. Type '\c' to clear the current input statement.

mysql> help 'contents'
You asked for help about help category: "Contents"
For more information, type 'help <item>', where <item> is one of the following
categories:
   Account Management
   Administration
   Compound Statements
   Data Definition
   Data Manipulation
   Data Types
   Functions
   Functions and Modifiers for Use with GROUP BY
   Geographic Features
   Help Metadata
   Language Structure
   Plugins
   Procedures
   Table Maintenance
   Transactions
   User-Defined Functions
   Utility

mysql>
mysql>
mysql>
mysql> help contents
You asked for help about help category: "Contents"
For more information, type 'help <item>', where <item> is one of the following
categories:
   Account Management
   Administration
   Compound Statements
   Data Definition
   Data Manipulation
   Data Types
   Functions
   Functions and Modifiers for Use with GROUP BY
   Geographic Features
   Help Metadata
   Language Structure
   Plugins
   Procedures
   Table Maintenance
   Transactions
   User-Defined Functions
   Utility

mysql>

===============================================================
Lastly, even without the patch,  calling  help 'contents' didn't eat up any additional rows as in the description  (2 rows are deleted instead of 1).